1. Field of the Invention
The present invention relates to an apparatus for supplying bottom-blown gas through a porous plug in ladle for molten metal refining.
2. Description of Related Art
In the process of refining molten metal, the molten metal subjected to primary refining in a refining furnace is transferred to a ladle, wherein a secondary refining is usually carried out. This is called ladle refining. A porous plug, which usually consists of a refractory, is provided at the bottom of the ladle. During refining, an inert gas such as argon or nitrogen gas is blown through this porous plug to agitate the molten metal.
FIG. 6 shows a porous plug and a supply pipe for inert gas disposed at the bottom of a conventional ladle. A porous plug 2 for bottom-blown gas is installed at the bottom of a ladle 1, and the tip end of the gas supply pipe is connected to the porous plug 2 via a cap 7.
The rear end of the gas supply pipe 3 is connected to or separated from a supply source 6 by a joint 5. When a switching valve 4 is opened and connected to the supply source, the bottom-blown gas is blown through the porous plug 2 to agitate the molten metal 8. After the refining is finished, the switching valve 4 is closed, by which the flow of gas (i.e., bottom blowing) is stopped, and the gas supply pipe 3 is separated from the gas supply source 6 at the joint 5, so that the ladle 1 is free to be moved.
After the ladle 1 filled with molten metal is moved to its destination the molten metal is discharged from the ladle for casting. The molten metal is discharged by opening a nozzle (not shown) provided at the bottom of the ladle to allow the molten metal to flow out for casting. The porous plug 2 has a porous construction such that a great number of ventilating pores are open. Just after the switching valve 4 is closed, the molten metal is prevented from permeating the ventilating pores by the pressure of gas remaining in the gas supply pipe 3. However, the gas pressure is soon decreased by the consumption of gas and decrease in temperature, so that the permeation of molten metal remaining in the ladle occurs.
The permeated molten metal cools and solidifies during the preparation for receiving the next charge, e.g., during the removal of slag etc. in the ladle performed after the transfer of molten metal. Once the metal is solidified, the ventilating pores are clogged. It is difficult to remove the solidified metal and gas blowing becomes impossible. Therefore, conventionally, the porous plug must be replaced for each charge.
To prevent the clogging, an apparatus has been proposed in which an accumulator tank is provided in the gas supply pipe to delay the decrease in gas pressure. For example, Unexamined Japanese Utility Model No. 64-15656 discloses a piping circuit for bottom-blown gas shown in FIG. 5. In this piping circuit, an accumulator tank 11 with a check valve 12 is connected in series with a gas supply pipe 3. Even after the gas supply pipe 3 is separated from the gas supply source 6, an amount of gas remains in the accumulator tank 11, so that a sudden decrease in gas pressure does not occur.
However, in the above piping circuit for bottom-blown gas in which the accumulator tank is connected in series with the gas supply pipe, the accumulated pressure does not exceed the pressure during blowing, and the capacity of the accumulator tank is not so large. Therefore, the permeation of molten metal into pores of the porous plug occurs even before the start of transfer of molten metal; this causes the ventilating pores to be clogged. This poses a problem in that the effect of the use of the accumulator tank is minimal; the porous plug can be reused two or three times only.